Consistent Crash using Cinematic Viewport on Linux

We are getting multiple crashes on 5.8.1 when trying to use the cinematic viewport on Linux, the most consistent way it's been reproduced is by clicking on the blue frame number to the left of the cinematic viewport transport controls. After some investigation, we believe this is a Linux specific memory handling issue between two duplicate definitions of the SNonThrottledSpinBox type between MovieSceneTools and LevelSequenceEditor. We've also seen crashes less reliably when closing the editor with the cinematic viewport active, and we believe this memory issue is the same root cause.

Steps to Reproduce

• Open the editor on Linux (any project, no repro project necessary)
• Open the cinematic viewport
• Click the blue frame number to the left of the viewport transport controls

Additionally, closing the editor with the cinematic viewport active can also crash the editor

0x00007ff1756f8a49 libUnrealEditor-Core.so!FDebug::CheckVerifyFailedImpl2(char const*, char const*, int, char16_t const*, ...) [/mnt/horde/++UE5/Sync/Engine/Source/./Runtime/Core/Private/Misc/AssertionMacros.cpp:793]
0x00007ff1507edb5d libUnrealEditor-MovieSceneTools.so!SNonThrottledSpinBox::OnFocusReceived(FGeometry const&, FFocusEvent const&) [/mnt/horde/++UE5/Sync/Engine/Source/Editor/MovieSceneTools/Private/CurveKeyEditors/SNumericKeyEditor.h:32]
0x00007ff16cd4f12c libUnrealEditor-Slate.so!FSlateApplication::SetUserFocus(FSlateUser&, FWidgetPath const&, EFocusCause) [/mnt/horde/++UE5/Sync/Engine/Source/./Runtime/Slate/Private/Framework/Application/SlateApplication.cpp:3167]
0x00007ff16cd3926f libUnrealEditor-Slate.so!FSlateApplication::SetUserFocus(unsigned int, TSharedPtr const&, EFocusCause) [/mnt/horde/++UE5/Sync/Engine/Source/./Runtime/Slate/Private/Framework/Application/SlateApplication.cpp:2730]
0x00007ff16cd4cf4a libUnrealEditor-Slate.so!FSlateApplication::ProcessReply(FWidgetPath const&, FReply const&, FWidgetPath const*, FPointerEvent const*, unsigned int) [/mnt/horde/++UE5/Sync/Engine/Source/./Runtime/Slate/Private/Framework/Application/SlateUser.cpp:204]
0x00007ff16cd633a7 libUnrealEditor-Slate.so!FSlateApplication::RoutePointerDownEvent(FWidgetPath const&, FPointerEvent const&) [/mnt/horde/++UE5/Sync/Engine/Source/./Runtime/Slate/Private/Framework/Application/SlateApplication.cpp:493]
0x00007ff16cd302cd libUnrealEditor-Slate.so!FSlateApplication::ProcessMouseButtonDownEvent(TSharedPtr const&, FPointerEvent const&) [/mnt/horde/++UE5/Sync/Engine/Source/./Runtime/Slate/Private/Framework/Application/SlateApplication.cpp:5391]
0x00007ff16cd62667 libUnrealEditor-Slate.so!FSlateApplication::OnMouseDown(TSharedPtr const&, EMouseButtons::Type, UE::Math::TVector2) [/mnt/horde/++UE5/Sync/Engine/Source/./Runtime/Slate/Private/Framework/Application/SlateApplication.cpp:5289]
0x00007ff16d3ba666 libUnrealEditor-ApplicationCore.so!FLinuxApplication::ProcessDeferredMessage(SDL_Event) [/mnt/horde/++UE5/Sync/Engine/Source/./Runtime/ApplicationCore/Private/Linux/LinuxApplication.cpp:535]
0x00007ff16d3b8647 libUnrealEditor-ApplicationCore.so!FLinuxPlatformApplicationMisc::PumpMessages(bool) [/mnt/horde/++UE5/Sync/Engine/Source/./Runtime/ApplicationCore/Private/Linux/LinuxApplication.cpp:284]
0x000000000023870f UnrealEditor!FEngineLoop::Tick() [/mnt/horde/++UE5/Sync/Engine/Source/./Runtime/Launch/Private/LaunchEngineLoop.cpp:5784]
0x000000000023d4d7 UnrealEditor!GuardedMain(char16_t const*) [/mnt/horde/++UE5/Sync/Engine/Source/./Runtime/Launch/Private/Launch.cpp:60]
0x00007ff168e11fbd libUnrealEditor-UnixCommonStartup.so!CommonUnixMain(int, char*, int (char16_t const), void ()) [/mnt/horde/++UE5/Sync/Engine/Source/./Runtime/Unix/UnixCommonStartup/Private/UnixCommonStartup.cpp:324]
0x00007ff168b1dd90 libc.so.6!UnknownFunction(0x29d8f)
0x00007ff168b1de40 libc.so.6!__libc_start_main(+0x7f)
0x0000000000234dfa UnrealEditor!_start(+0x29)